Initiative is how quickly your character can respond to a crisis situation, typically combat. Because people react to a situation differently, each character will have a unique initiative value. Special units that have received more training than normal, such as Scout/Snipers or Recon, get an additional modifier to their initiative to reflect a better ability to think in stressful situations; conversely, "paper-pushers" such as Intelligence and Supply lose some initiative because of a lack of combat-type situations.
Each character has a base level of initiative, plus any inherent modifiers. This value is then added to a random "die-roll" every round to account for individual and situational changes. In certain situations, initiative doesn't need to be determined when it's obvious who has the upper hand, e.g. an ambush or a sniper waiting for a target.
To determine the base initiative level, each character rolls 1d6, with +1 if the character's Primary MOS is a Sniper/Scout, Recon, or Aircraft Pilot and -1 if the Primary MOS is Supply, Intelligence, or Medical.
During combat or other stressful situation (as determined by the Game Master), individual initiative is determined by rolling 1d10 for each character and adding the base initiative. This also applies to any NPC's (non-playing characters) such as enemies or allies.